FIG. 5 is a block diagram of a conventional vector control apparatus. The vector control apparatus includes a three-phase alternating current source, a rectifying circuit 2 including a diode, etc. for obtaining direct current voltage from the three-phase alternating current source 1, an electrolytic capacitor 3 that smoothens the direct current voltage, an inverter circuit 4 including switching elements such as transistor, etc., an induction electric motor (hereinafter “a motor”) 5 to which a load is connected, a speed detector 6 that detects the rotation speed of the motor 5, a current detector 7 that detects the three-phase primary current Iu, Iv, and Iw flowing into the motor 5, a speed command circuit 8 that assigns a speed instruction ω* of the motor 5, a three-phase-to-two-phase current converter 9 that calculates an excitation current I1d and a torque current I1q from the three-phase current Iu, Iv, and Iw, a vector control calculating circuit 10 that calculates primary voltage instruction values V1d* and V1q* to be assigned to the motor by inputting the speed instruction value ω* of the speed instruction circuit 8 and a detection value ω of the speed detector 6 as well as the two-phase calculation values I1d and I1q of the current detector 7, and a two-phase-to-three-phase voltage converter 11 that calculates three-phase output voltage instruction values Vu*, Vv*, and Vw* from the two-phase primary current instruction values V1d* and V1q*. 
The vector control apparatus further includes an output torque estimator 12 that calculates an output torque of the motor 5 from the calculation values I1d and I1q of the three-phase to two-phase current converter 9, an analog output unit 13 that digital-to-analog converts the output torque estimation value of the output torque estimator 12 and outputs an analog voltage, and an analog input unit 14 that analog-to-digital converts the analog voltage signal and converts the digital analog voltage signal to a torque instruction. In the circuit shown in FIG. 5, although all three, namely, the output torque estimator 12, the analog output unit 13, and the analog input unit 14 are present, if only a master is involved, the output torque estimator 12 and the analog output unit 13 are necessary, and if only a slave is involved, the analog input unit 14 alone is necessary.
Explained next is a control method for the synchronous operation of two vector control inverters that have the structure described above.
In the synchronous operation, given that q axis current I2q is controlled such that it is zero as regards the flux of a secondary side rotor, the following expression (1) is used for calculating the output torque by the output torque estimator 12 based on the calculation result of the three-phase-to-two-phase current converter 9 in the master vector control inverter.Tm=Kt·I1q·I1d  (1)where Kt is a torque coefficient corresponding to the motor.
In the analog output circuit 13, the digital value is converted into an analog value such that the calculation result of the output torque estimator 12 is normalized to match the bit count of the analog-to-digital converter in the analog input circuit 14 on the slave side and an analog voltage is output to the slave vector control inverter.
In the slave vector control inverter, the analog voltage that is output from the master vector control inverter is input to the analog input circuit 14 and converted into a torque instruction and the motor 5 is rotated in a torque control mode.
Thus, in the synchronous operation that employs the conventional vector control inverters, the torque is estimated by the master vector control inverter, the estimated digital torque value is once converted to an analog signal and output to the slave vector control inverter. The analog signal received from the master vector control inverter is converted into a digital torque value by the slave vector control inverter and the motor 5 on the slave side is rotated in the torque control mode. The transfer for synchronization signals involves conversion of a digital value to an analog signal (a process that takes place in the master vector control inverter) and conversion of the analog signal back to the digital value (a process that takes place in the slave vector control inverter). Therefore, any offset in the analog signal or a fluctuation in the level of the analog signal affects functioning of both the master side and the slave side (for instance, discord or fluctuation, etc. of the slave side with regard to the master side). Besides, since an analog signal is used between the master side and the slave side, the noise factor also casts a considerable effect on the functioning of the master side and the slave side.
As an alternative method, data is transferred as a digital signal by employing a serial communication network between the master and the slave. In this case, for the synchronous operation, it is necessary to transmit the torque signal of the master vector control inverter in realtime to the slave vector control inverter. In the case where plural slave vector control inverters are used, the torque signal of the master vector control inverter is required to be transmitted to all the slave control inverters simultaneously.
Consequently, carrying out the synchronous operation in a serial communication network necessitates a complex system with requirement of communication control hardware for fast data transmission between the master and the slave, and communication software for receiving signals for obtaining synchronization between the inverters and for the inverters to receive data and carry out processes in accordance with the synchronization signals.
In Japanese Patent Laid Open Publication No. H9-182481, a speed difference control apparatus is disclosed that, using a pulse array control, drives a slave servo motor to rotate at a predetermined speed difference with respect to the rotation speed of a master servo motor. This speed difference control apparatus pulse array controls the rotation speed of the slave servo motor based on the sum or difference of the detected pulse array frequency value of the rotation speed of the master servo motor and the pulse array frequency of the predetermined speed difference. However, in this method, synchronous operation is possible only if the structure comprises a single master servo motor and a single slave servo motor. In a structure that comprises two or more slave servo motors, it is not possible to keep the conditions identical, since the sum or difference are obtained for each slave servo motor with respect to the master servo motor.
In Japanese Patent Laid Open Publication No. H11-41967, a driving apparatus in the form of an operation control apparatus that includes plural rotation-driven wheels is disclosed. This operation control apparatus includes a speed control mode inverter that speed-controls one of the wheels based on the operation speed set by a target speed setting unit and a torque control mode inverter that produces a torque equal to that of the speed control mode inverter and torque-controls the wheels excluding the wheel that is speed-controlled by the speed control mode inverter. However, this conventional technology does not allow removal or addition of a slave axis during the synchronous operation.
Therefore, it is an object of the present invention to obtain a vector control inverter that allows transfer of synchronization signals (digital signals) without the necessity for a conversion process from digital signals to analog signals and vice versa.
It is another object of the present invention to allow synchronous operation in a system comprising a single master vector control inverter and two or more slave vector control inverters, and further allow removal or addition of slave axes during synchronous operation.